1. Field of the Invention The present invention relates to a vibration actuator. More specifically, the present invention relates to a vibration actuator that generates relative motion between a relative moving member pressed in contact with a vibration element by exciting an elastic member by an electromechanical converting element and generating elliptic motion on the surface of the vibration element by harmonically generating a plurality of vibration modes.
2. Description of Related Art
Conventional vibration actuators generate longitudinal vibration and bending vibration on an elastic member as well as generate elliptic motion on the surface of an elastic member by joining an electromechanical converting element to the elastic member and applying an alternating-current voltage to this electromechanical converting element.
The structure and load properties of such a vibration actuator have been explained in "Piezoelectric linear motor for light pickup movement" (Yoshio TOMIKAWA, et al.: Proceedings of the Fifth Electromagnetic Power Dynamics Symposium), the subject matter of which is incorporated herein by reference.
FIG. 15 shows the configuration of such a vibration actuator 1. The vibration actuator 1 is rectangular and planar and has an elastic member 2 on one side on which driving force output members 2a and 2b are formed in protruding shapes. A relative moving member is pressed into contact with the elastic member 2 by the driving force output members 2a and 2b using an unillustrated pressing mechanism. Driving force output members 2a and 2b are formed in two locations to become the antinodes of a fourth-order bending vibration generated by elastic member 2 as will be described below.
A piezoelectric element 4 is formed on the other side of the elastic member 2 as a rectangular thin strip-shaped electromechanical converting element. Electrodes 5a, 5b, 5p and 5p' are formed on the surface of the piezoelectric element 4 in an electrically insulated state independent of each other.
An alternating-current voltage is applied to electrodes 5a and 5b with an electrical phase that differs by 90.degree. for each of the electrodes 5a and 5b. Electrodes 5p and 5p' detect the state of the vibration generated by the elastic member 2. Lead wires (not shown) are soldered to the electrodes 5a, 5b, 5p, and 5p' and to a control circuit (not shown).
Drive signals applied as alternating-current voltage to electrodes 5a and 5b generate a first-order longitudinal vibration and a fourth-order bending vibration harmonically in elastic member 2. An elliptic motion is generated at the end of driving force output members 2a and 2b. Relative moving member 3 is thereby pressed into contact with elastic member 2 by a driving force output member which performs relative motion against the elastic member 2. This relative motion is used as a thrust output.
Such a vibration actuator 1 is designed so that the inherent vibration frequencies of the first-order longitudinal vibration and the fourth-order bending vibration become extremely near or equal to each other. Therefore, by applying alternating-current voltage of a frequency near the two inherent frequencies on electrodes 5a and 5b, the first-order longitudinal vibration and the fourth-order bending vibration can be generated harmonically.
Sliding members 6a and 6b are glued on the end faces of the driving force output members 2a and 2b for reducing the sliding resistance with relative moving member 3. These sliding members 6a and 6b are generally made of resinous materials and metallic materials.
This vibration actuator 1 has a characteristic of having no risk of inductive damage due to electromagnetic induction because it does not use a magnetic field. However, long-term driving in a vacuum is not possible because when the sliding members 6a and 6b are made of metal, the generation of abrasion powder by sliding with the relative moving member 3 becomes severe. On the other hand, gas is emitted when the sliding members 6a and 6b are made of resinous material.
Vibration actuator 1 substantially operates by the frictional driving between the affixed sliding members 6a and 6b and the relative moving member 3. Also, the achieved driving force depends on the product (.mu..times.W) of the pressure W and the coefficient of friction .mu. between the elastic member 2 and the relative moving member 3.
The driving force (.mu..times.W) can be raised by increasing the pressure W. However, if the pressure W is too great, the expanding and contracting generated in the elastic member 2 is suppressed and conversely the driving force decreases.
If the coefficient of friction .mu. of the sliding members 6a and 6b is made higher, the driving force (.mu..times.W) can be raised. However, when the abrasion-resistance of sliding members 6a and 6b is great, the elastic member 2 causes defacement of the sliding surfaces of the elastic member 2 composed of material having a specified elasticity (for example, iron, stainless steel, aluminum alloys, etc.) and relative moving member 3. Thus, drive control as the initial stage of driving becomes extremely difficult. It could also be used only in an environment where the generation of abrasion powder does not become a problem.
Furthermore, there is a problem that the roughness occurring on the sliding surfaces of the elastic member 2 and the relative moving member 3 due to defacement of the sliding surfaces becomes a stimulus for wear of the sliding members 6a and 6b and it decreases the life of vibration actuator 1.
Further yet, the vibration actuator 1 does not tend to generate noise. However, noise is generated if metallic sliding members are used. Also, if resinous sliding members are used, the resin itself absorbs the vibration of the elastic member 2 depending on the type of resin and the driving force decreases. Furthermore, if resinous sliding members are used, the abrasion powder of the resin attaches to the other material in contact, the attached abrasion powder and the sliding members harden and become inseparable, and the driving of the vibration actuator stops. Also, noise is generated according to the roughness average of the contact areas of the vibration element and the relative moving member 2 contacted by a sliding member made of metallic material or resinous material.